Shape the dumplings: Sprinkle a small amount of flour on a counter or silicone rolling mat. Spoon about 1/4 of the dough onto your rolling surface. Sprinkle your hands with flour and roll it out into a long rope, about 1 - 1 1/2 inch thick (3-4cm) sprinkling with more flour if necessary. Cut at an angle with a knife dipped in flour or a bench scraper into 2-inch (5cm) dumplings. Repeat with the remaining dough.
Cook the leniwe: Bring a large pot of salted water to a boil. Cook the leniwe over low heat in 4 batches. Add the first batch of dumplings to the pot, stir them gently with a wooden spoon to prevent them from sticking to the bottom of the pot. After they float to the surface, cook them for about 2 minutes. Make sure the water is simmering gently and not boiling rapidly or the dumplings may fall apart.
Transfer to a plate with a slotted spoon and toss with butter to prevent sticking.
Make buttered breadcrumbs (bułka tarta): Melt the butter in a pan, add the breadcrumbs and sugar. Cook, stirring over medium heat until breadcrumbs are golden in color (2-3 minutes).
Serve the dumpling with buttered breadcrumbs, sprinkle with more sugar (optional).
Enjoy!